1. Field of the Invention
The present invention relates to an optical sensor comprising an amorphous silicon layer with a greater light absorption coefficient for visible light to produce photo carriers for use as a photo current to be transmitted via highly mobile polycrystal silicon. More specifically, the present invention relates to an optical sensor comprising an amorphous silicon layer formed to bring into contact with a channel forming region of a bottom gate-type polycrystal silicon thin film transistor.
2. Description of the Related Art
Optical sensors are commonly used as linear image sensors or area image sensors that convert images produced by facsimiles, copiers, video cameras, digital still cameras, and the like into electrical signals. As materials for optical sensors single crystal silicon, or an amorphous silicon layer are employed. However, except in extraordinary cases, since images produced in a wavelength range of visible light are converted into electrical signals in most cases, the amorphous silicon layer with a greater light absorption coefficient for visible light is commonly used.
Optical sensors using amorphous silicon are divided into two major types: 1) a resistance-type and 2) a diode-type. In the resistance type, a greater current can be obtained due to amplification action as a transistor. However, since it produces carriers in large quantities by amplification, annihilation or collection of the amplified carriers are virtually impossible even after the light is interrupted, which results in a slower light response rate and narrower dynamic range controlled by the intensity of light. In the diode type, there is a feature that a depletion layer spreads widely in the amorphous silicon, thereby allowing the photo carriers produced upon the incidence of light to be easily collected, and a faster light response rate due to the lack of amplification action and a wider dynamic range controlled by the intensity of light are obtained. However, because the current is small in the diode-type, a capacitor is required for retaining electric charges.
A switch to output signals detected by an optical sensor as an output signal with time-division has a bare IC-type that utilizes a field effect transistor of a single crystal semiconductor (mainly, silicon semiconductor) as an analog switch. Analog switches include TFT-types using a thin film transistor that employs amorphous silicon or polycrystal silicon for a channel forming region.
The IC types have a faster switching rate and a greater performance reliability but require as many analog switches as optical sensors as the bare IC chip, resulting in high cost switch applications. At the same time, since both a thin film substrate used to form a light absorption layer (optical sensor portion) such as amorphous silicon and the bare IC chip are required, the area thereof becomes wider, thereby being an obstacle of a size reduction. The TFT-types utilize thin films for forming the switches to allow both the optical absorption layer such as amorphous silicon and a TFT for use as a switch to be formed on the same substrate, thereby being capable of easily reducing the area and downsizing, and drastically cutting the costs compared to the IC-types. Among the TFT-types, thin film transistors using amorphous silicon for the channel forming region (amorphous silicon TFTs) utilize the amorphous silicon TFTs also for forming the switch element when, for example, the optical sensor portion is formed of amorphous silicon, resulting in lower cost than when utilizing polycrystal silicon TFTs due to sharing of the fabrication process. However, a faster switching rate is impossible due to the mobility of the amorphous silicon as small as 1 cm.sup.2 /Vsec. For that reason, amorphous silicon is not applicable for an area sensor where area elements are increased in number, and for a linear sensor capable of dealing with high-speed.
Among the TFT-types, thin film transistors using the polycrystal silicon for forming the channel forming region (polycrystal silicon TFTs) require the formation of polycrystal silicon in addition to the formation of the light absorption layer such as amorphous silicon, resulting in more fabrication processes than in the case where the amorphous silicon TFTs are used. However, since the mobility of the polycrystal silicon is as great as 10 to 200 cm.sup.2 /Vsec, a faster switching rate is possible. For that reason, an image sensor comprising the optical sensor element formed by the amorphous silicon and the switch element formed by the polycrystal silicon TFT is effective.
Most image sensors comprising the optical sensor element formed by amorphous silicon and the switch element formed by polycrystal silicon combine a diode type optical sensor utilizing the amorphous silicon and polycrystal silicon TFT for use as separate devices. The reason is that the use of the resistance-type optical sensor results in a reduced response rate, thereby making full use of the high-speed switching capabilities of the polycrystal silicon TFT impossible.
In most cases, either a 1) p-i-n diode or 2) Schottky diode is used for forming the diode-type amorphous silicon optical sensor element. The p-i-n diode forms a triple electro conductive layer of the p, i, and n types, where a depletion layer extends in the i-type amorphous silicon region, to thereby allow electrons to be transmitted into the n-type region and holes into the p-type region, with almost no recombination of the photo carriers produced therein.
The p-i-n diode-type structurally requires either the p-type or the n-type, utilizing silicon carbide (SiC), microcrystal silicon (.mu.c-Si), silicon nitride (SiN), and the like. The p-type and n-type layers require binary to quadrinary reaction gases, making the entire fabrication process complicated.
The Schottky diode-type forms a Schottky barrier by putting the amorphous silicon in contact with non-ohmic contact type conductive materials at the position thereof to use the resulting depletion layer formed in the Schottky barrier. The Schottky barrier is formed by simply forming a conductive film, which is much easier than the p-i-n diode-type. However, the depletion layer is formed in a narrower area compared to the p-i-n diode-type, making full collection of produced photo carriers difficult. A thinner amorphous silicon layer is required to collect all the photo carriers produced but has to tolerate smaller photo carrier quantities produced due to inferior optical absorption performance, resulting in lower photosensitivity for an optical sensor. A thicker amorphous silicon layer to increase optical absorption performance prevents the depletion layer from extending in the entire amorphous silicon and generates a resistance portion inside. That makes impossible to collect the produced photo carriers to recombine the same.
Either the p-i-n diode-type or the Schottky diode-type delivers a greater light absorption coefficient in shorter wavelengths of 450 nm and below when absorbing visible light to produce photo carriers, which causes the light to be absorbed before it reaches the depletion layer of the diode. This triggers recombination of the photo carriers produced by shorter wavelengths before reaching the depletion layer, resulting in no electrical signal outputs, which indicates weak sensitivity of blue color of an optical sensor.
In either of the Schottky diode-type or the p-i-n diode-type, the optical sensor element formed of the amorphous silicon and the polycrystal silicon TFT portion are formed in different locations. Therefore, upon the fabrication of the area sensor, the sensor portion and the TFT portion are formed in a single element. As a result, the region area of the light absorption element such as amorphous silicon and the like, which actually absorbs light, is reduced, making large-volume optoelectrical signal receptions difficult.